The invention relates to a thrust-control device for a motor vehicle having a dual circuit brake system with a drive slip control system (ASR) effective during driving of the wheels and an anti-locking system (ABS) effective during braking of the wheels.
A device of this type is described in German Published Unexamined Application (Offenlegungsschrift) No. 3,137,287. It is intended for a vehicle with wheels driven on one axle.
Thrust-control devices are also known for vehicles with all-wheel drive.
Thus, for example, the previously published German Offenlegungsschrift No. 3,421,776 describes a multiple-circuit braking system in which the operation of the drive-slip control (ASR) can be superposed on the operation of the anti-lock system (ABS). In this case, the pressure accumulator serves also for the purpose of intensifying the braking pressure in the course of pedal-controlled braking. In the case of the drive-slip control, the main cylinder is pressurized by means of a solenoid valve connected upstream. Such pressure is then transmitted dynamically to the wheels of one axle. The other axle is placed under static pressure by means of a main cylinder piston. The magnitude of the pressure at the wheel brakes is determined by the control valves associated with the wheels.
In order to prevent exhaustion of the static brake circuit, brake fluid can be supplied via a reversing valve from a dynamic circuit.
A problem with braking systems of this kind is that, in the event of failure of the external pressure source, not only the intensification of braking pressure fails; the dynamic circuit can also no longer be pressurized. In addition to this, the ABS and ASR controls are also affected by the failure of the external pressure source. Their operation cannot be maintained any longer.
Accordingly, the coupling involves serious technical problems, since the driver is surprised by unusual behavior of the vehicle.
In contrast to this, the object of the invention is to develop a thrust-control device which uses a special auxiliary pressure source (vacuum source) for the brake booster and which provides a special pressure accumulator for the thrust control, in such a way that the arrangement can also be used for the thrust control of vehicles with all-wheel drive.
According to the invention this object is achieved by providing a pressure accumulator operative during drive-slip control operation of the vehicle for controlling the spinning of the wheels and a stop valve and control valve for controlling the connection between the pressure accumulator and the wheels of the axle that is not directly driven in a two wheel drive motor vehicle. In the case of two wheel drive, the stop valve and control valve connect the main brake cylinder and the wheels not directly driven. In a four wheel drive, the stop valve and control valve connect the pressure accumulator and the wheels that are now directly driven. This same arrangement may be used for both two and four wheel drive vehicles.
One particular advantage of the invention is that a device for a single-axle drive can be extended by only small additional means to vehicles with a multi-axle drive, without requiring an additional pressure accumulator and while preserving the independence of the brake circuits. If the pressure accumulator fails, only the drive-slip control is affected, whereas normal braking and anti-lock braking continue to function.
Further objects, features and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings, which show for purpose of illustration only, an embodiment constructed in accordance with the present invention.